In combustion chambers of gas turbines fuel is burned for generating thermal energy. The combustion chamber comprises a burner body with a pilot burner face which comprises a liquid fuel lance having a conduit for guiding the liquid fuel to a tip, the tip for injecting pilot fuel and holes in the tip for injecting cooling air. The holes for injecting cooling air are generally formed in and around the liquid fuel tip. The cooling air may be guided along the liquid fuel lance to its tip in an annulus between the liquid fuel lance and the bore through the burner body in which it is installed. The cooling air is normally supplied from the gas turbine compressor discharge utilizing the same available pressure drop as the main flow through the burner, however flowing in a parallel stream for the two flows to be joined in the burner cavity. The upstream wall of the burner cavity, i.e. the pilot burner face, may reach temperatures between approximately 800°-1000° C. (Celsius) during operation. The holes for injecting cooling air cools the lance tip and the injected cooling medium interacts with the fuel injected from the lance tip to create a homogeneous air/fuel mixture. In order to achieve a combustion with low emissions, it is a need to achieve a high degree of atomization of the fuel in a main operating range. During a start-up phase and during low load operation of the gas turbine, a less distributed fuel spray is generated, due to the reduced mass flow and pressure, which increases the tendency of the fuel to deposit on the lance tip and the adjacent areas. During operation the fuel covered surfaces on the pilot burner surface may coke and carbonize, such that a hard and adhesive coating is generated. This coke and carbonization onto the pilot burner face may lead to a blockage of the holes for injecting the cooling medium. Hence, the temperature of the lance tip may increase and the fuel flow through the lance tip may ultimately stop if the fuel orifice of the lance tip is blocked by carbonized fuel.
FIG. 5 illustrates a common combustion chamber 500 comprising a burner body 110 with a common wall section 501. Through the common wall section 501, a pilot fuel is injectable along an axial direction of the common combustion chamber 500. Through the common wall section 501, the pilot fuel and a cooling medium is injectable in a predefined direction 107. Moreover, an igniter 502 is attached to the burner body 110 in order to ignite the injected fuel during start-up.
In surrounding shell sections of the common combustion chamber 500 a swirler 503 is formed, wherein the swirler 503 is adapted for injecting a main fuel/air stream 504 in a circumferential direction. The injected pilot liquid fuel stream and the injected cooling medium are injected for controlling the combustion of the main fuel/air mixture stream 504 which flows through the swirler 503 of the common combustion chamber 500.
As shown in more detail in FIG. 6, the common wall section 501, which may be a part of a fuel lance that is inserted in the pilot burner body 110, comprises common inlet holes 601 that are formed circumferentially around a fuel injection aperture 106 as to promote the characteristics of the spray. Through the fuel injection aperture 106 the pilot fuel is injected into the common combustion chamber 500 in a predefined direction 109. The common inlet holes 601 have a cross-section through which cooling medium is injected which interacts with the pilot fuel injected in the direction 109 through the fuel injection aperture 106 of the pilot lane. Hence, the entered fuel may carbonize inside the common inlet holes 601 due to the high temperature inside the common combustion chamber 500 and thus block the common inlet holes 601.
U.S. Pat. No. 5,833,141 A discloses an anti-coking dual-fuel nozzle for a gas turbine combustor. The dual-fuel nozzle comprises a liquid fuel nozzle surrounded by an air/gas pre-mixing cup. The cup has a base comprised of swirler vanes surrounding the outer tube of the liquid fuel nozzle. The air/gas cup surrounds the liquid fuel nozzle such that channels between the liquid fuel nozzle and the air/gas pre-mixing cup are formed. In a downstream end section of the air/gas pre-mixing cup, the air/gas pre-mixing cup comprises a conical section.
U.S. Pat. No. 6,123,273 A discloses a dual-fuel nozzle for inhibiting carbon deposition onto combustor surfaces in a gas turbine. The dual-fuel nozzle comprises a liquid fuel nozzle surrounded by a gas fuel nozzle. A converging sleeve surrounds the converging outer wall of the combined liquid fuel and gaseous nozzle to form a duct of decreasing cross-sectional areas in a downstream direction, such that an airflow through the duct accelerates towards the conical droplet spray pattern emerging from the liquid fuel nozzle. The accelerated air flowing through the duct precludes an impingement of oil spray droplets onto metal surfaces of the nozzle.
In other approaches, the critical surfaces of the elements of a gas turbine that are in contact with fuel are coated with high temperature alloys with a coke inhibiting layer.